shell: succeed with a `not_` versus `nil` streams.

* specs/shell_spec.yaml: Specify behaviours with a `not_` matcher
against a `nil` stream.
* specs/inprocess_spec.yaml (capture): Likewise.
* lib/specl/shell.lua (contain_output, succeed_while_containing)
(output, match_output, succeed_while_matching, fail)
(contain_error, fail_while_containing, output_error)
(match_error, fail_while_matching): Treat a `nil` valued stream
as "" before calling string.match on it!
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>